1188 Wisconsin Academy of Sciences, Arts and Letters. 
cells in the colony, the four-celled coenobes being much more 
common when the environment is less favorable. 
In colonies that have come from a common ancestry, as those 
in a pure culture, and which have been kept under the same 
cultural conditions all the time, there is considerable variation. 
The most striking difference is that the cells may be arranged 
in a linear series (Text Figure 7 A) or in an alternate or zig¬ 
zag arrangement (Text Figure 7 B). Associated with this 
variation in cellular arrangement is an eccentric arrangement of 
the pyrenoid. 
ture in 0.1% Knop’s solution, Figs. B & C from cultures in 1.0% Knop’s 
solution. 
When the cells have the alternate arrangement the pyrenoids 
are in,an alternate eccentric position, while when the cells are 
in a linear series the pyrenoids are eccentrically placed two 
by two. Besides these two general types there is a 
third which does not agree with the distinguishing 
character of the genus as described by Be Toni (39). Scene - 
desmus cells similar to those shown in Text Figure 7 C would 
not come under De Toni’s description (p. 563) ; “ ... .cellulae 
.in seriam simplicem vel subduplicem lateraliter con- 
junctae.” This type has the long axes of the cells in two planes, 
the long axes of two of the cells having revolved through an arc 
of 90 degrees. This type is quite rare and may sometimes con¬ 
sist of only three,cells (Fig. 42 A&B, Plate LXXXVXI) in¬ 
stead of the usual four. This arrangement may possibly he 
considered an extreme case of the deviating type shown in 
Figure 45. 
